This invention relates to a control apparatus for an automatic transmission having a torque converter. More particularly, the invention relates to measures for alleviating shock produced when a select lever is manually operated to make a range changeover from the R range to the D range, by way of example.
Several technical proposals have been made for a control apparatus for an automatic transmission of this type in Japanese Patent Application Laid-Open (KOKAI) Nos. 61-65947, 61-65948, 61-65949, and 61-65950 U.S. Pat. No. 4,694,709 corresponds to KOKAI Nos. 61-65947 and 61-65948, while U.S. Pat. No. 4,665,777 corresponds to KOKAI Nos. 61-65949 and 61-65950. The proposed control apparatus are adapted to mitigate shock produced when a changeover is made from the neutral (N) range to a traveling range (e.g, the D range).
The proposed control apparatus perform control as follows when the above-mentioned range changeover is made: When the N.fwdarw.D range changeover is made, the change ratio is temporarily set to a high speed (third speed, for example). Since the transmission is in the neutral state up to the moment of the speed change, the rotational frequency (rev count) N of the converter turbine should be a certain value (n.sub.0, for example) at the moment the change ratio is set. Accordingly, when the change ratio is set to a high speed, such as third speed, the turbine rotational frequency N drops from the value n.sub.0 owing to the load upon the gear mechanism of the transmission. The arrangement is such that a changeover is made to first speed at the moment the turbine rotational frequency N falls to a set value (e.g., n.sub.1). In other words, it is possible to gently raise the driving torque transmitted from the automatic transmission to the wheels by first setting the change ratio to a high speed (third speed) and then subsequently changing the ratio to first speed. Shock at the moment of the range changeover is thus mitigated effectively.
The range changeover operation includes changeovers other than the shift from neutral N to drive D. For example, a changeover frequently made is to shift from the reverse range R to the drive range D by rapidly passing through the neutral range N. Since this changeover operation entails shifting from reverse range to drive range via the neutral range, it includes what is referred to as an "N.fwdarw.D" exercise. Accordingly, if the afore-mentioned arrangement (Japanese Patent Application Laid-Open No. 61-65949), in which a high speed is traversed in order to prevent transmission shock at the N.fwdarw.D shift, functions as described, shock at the time of the R.fwdarw.N.fwdarw.D range changeover should be alleviated effectively.
In actuality, however, it has been found that there are instances in which the R.fwdarw.D changeover and D.fwdarw.R are accompanied by a comparatively large transmission shock.
Accordingly, the inventor has carried out extensive research and, as a result, has clarified the cause of this transmission shock. Specifically, the cause of shock is that, at the time of the R.fwdarw.D changeover, the turbine rotational frequency N falls to a value lower than the set rotational frequency value n.sub.1 which ought to prevail when the shift is made from the above-mentioned third speed to first speed.
More specifically, according to the above-mentioned prior changeover control apparatus the N.fwdarw.D or R.fwdarw.N.fwdarw.D changeover is made by first setting the change ratio to third speed and then altering the change ratio to first speed at the moment the turbine rotational frequency falls to n.sub.1 . This changeover rotational frequency is set to a low value owing to the requirement that the vehicle not be propelled forward in third speed. In the reverse range R prior to performing the changeover, stopping of the vehicle is accompanied by stopping the rotation of the turbine shaft, and therefore the rotational frequency is zero. When the R.fwdarw.D changeover is made, the change ratio is set to third speed and the turbine rotational frequency begins to rise, as mentioned above. Since n.sub.1 is set to a low value, the rotational frequency N promptly attains this changeover rotational frequency n.sub.1.
When control is performed in such a manner that the change ratio is altered to first speed merely by having the turbine rotational frequency N attain the changeover rotational frequency n.sub.1, the time for the gear mechanism of the transmission to be set to third speed is essentially shortened. As a consequence, control by way of traversing the high-speed change ratio, namely control performed to gently raise torque transmitted to the wheel, is essentially not carried out. The reason is as follows: In the gear mechanism of an automatic transmission, a fixed period of time is required in order for oil pressure to act upon clutches and brakes serving as friction elements and for these elements to essentially begin working. When, in spite of this, the time during which high speed such as third speed is in effect is short, third speed is maintained in a state in which these friction elements are not yet working. As a result, the torque transmitted to the wheels is essentially zero in third speed. Since a large torque acts upon the wheels once at the moment the change is made to first speed, torque shock becomes large in magnitude.
Though the R.fwdarw.D changeover is taken as an example in the foregoing, the reverse and forward operations are merely reversed when a D.fwdarw.R changeover is performed, and therefore the circumstances are similar to those which prevail at the time of the R.fwdarw.D changeover. That is, transmission shock is produced not only at the R.fwdarw.D changeover but also at the D.fwdarw.R changeover.